1. Field of the Invention
The present invention relates to a pad used for disk brakes that control the revolution of revolving parts of vehicles and machines.
2. Description of Background Information
Disk-brake pads are used for friction members in disk brakes that control the speed of automobiles, railway vehicles, and machines. The friction member comprises a backing plate that contacts a piston and a friction material that rubs against a disk. An underlayer material is sometimes provided between the backing plate and the friction material when the load for the friction material is severe or when the improvement of the performance of the friction material is required.
The underlayer material has three major functions. The first is to prevent the heat generated by friction from conducting to the brake piston. The second is to prevent the deterioration of the bonding between the friction material and the backing plate when the performance of the friction material is improved at the cost of bonding strength. The third is to reduce noises generated by the vibration of the friction material.
Therefore, it is desirable that the material used for the underlayer material not only be suitable for a friction material but also have a heat insulation effect, a good bonding with the backing plate, and an ability to compensate the performance weakness of a standard friction material.
A friction material particularly for heavy loads, known as a semimetallic pad, includes a substantial amount of steel fiber or steel particles. Because this pad has high thermal conductivity owing to its constituents and conducts the heat generated by friction to the backing plate readily, it is common to employ an underlayer material as a thermal insulator to prevent the heat conduction when this semimetallic pad is used.
In this case, the heat generated at the friction surface diffuses to both the disk and pad. When the pad has high thermal conductivity, it undesirably absorbs the heat. When the pad has low thermal conductivity, most of the heat is conducted to the disk and dissipated by the revolution of the disk. In other words, an underlayer material reduces the thermal conductivity of the pad and directs the generated heat to the revolving disk to be dissipated there.
An underlayer material should also have a strong bonding with the backing plate. In order to maintain the strong bonding in withstanding repetitive temperature variations, the underlayer material should have a composition similar to that of the friction material and thus have good compatibility with the friction material.
An underlayer material principally consists of fibrous materials, binders, and powder and/or granular fillers. Particularly, when a semimetallic material is used as the friction material, it is desirable that steel fiber be used as the fibrous material because the same has good compatibility with the friction material.
As the constituents thereof, some underlayer materials have the same phenolic resin-based binder as the friction material has and include a material having a cushion effect such as cashew dust or rubber powders to absorb vibration caused by friction. Because these organic substances have low thermal conductivity, small specific gravity, and small specific heat, they reduce the thermal conductivity of the underlayer material. A fibrous material is used to resist the shearing force generated between the disk rubbing surface and the backing plate when frictional force is applied. Inorganic fillers are used to reinforce heat-resisting properties.
As mentioned above, an underlayer material has various purposes, and to fulfill these purposes suitable materials are incorporated. However, under heavy load conditions, particularly when a car descends a long downhill with increased frequency of brake application, for instance, if the heat dissipation is insufficient from the brake, the heat generated by friction will accumulate in the disk and pad, raising the temperature thereof considerably. If this condition continues for a prolonged period of time or recurs many times, the organic materials used in the underlayer material will deteriorate and even carbonize in an extreme case, resulting in increased thermal conductivity and loss of the vibration absorption effect thereof.